Liquid crystal display devices have various advantages such as low power consumption, light weight, and slim profile. Owing to these advantages, they have taken the place of conventional CRT displays and have rapidly spread in these days.
Such a liquid crystal display device is provided with a polarizing element on the image display screen of its liquid crystal cell, and the polarizing element usually requires hardness so as to prevent damages when handled. In general, such hardness is imparted to an image display screen by providing, as a polarizer-protecting film, a hard coat film that includes a light-transmitting substrate and a hard coat layer disposed on the substrate.
Conventionally used light-transmitting substrates for such a hard coat film are cellulose ester films such as triacetyl cellulose films. This is based on the following advantages of the cellulose esters; that is, the cellulose esters have excellent transparency and optical isotropy and have little in-plane phase difference (have a low retardation value), and thus they hardly change the vibration direction of incident linearly polarized light and they have less influence on the display quality of liquid crystal display devices. In addition, the cellulose esters have appropriate water permeability. Thus, when a polarizer comprising an optical layered body is produced, the water remained in the polarizer can be dried through the optical layered body.
Such a cellulose ester film also has insufficient moisture and heat resistance. Such insufficient characteristics thus deteriorate the properties of the polarizer such as polarizing function and color phase when the hard coat film is used as a polarizer-protecting film in a high-temperature and high-humidity environment. Further, a cellulose ester is a disadvantageous material in terms of cost.
Such disadvantages of the cellulose ester films provide a demand for using, as a light-transmitting substrate of an optical layered body, a versatile film that has excellent transparency, heat resistance, and mechanical strength and can be more easily and more inexpensively available in the market than cellulose ester films or can be produced by a simpler method. For example, a polyester film such as polyethylene terephthalate is experimentally used as a substitution for a cellulose ester film (e.g. see Patent Literature 1 to 3).
However, a polyester film has a high-polarizability aromatic ring in its molecular chain and thus has very high intrinsic birefringence. Accordingly, the polyester film has a characteristic of easily exhibiting birefringence accompanying orientation of the molecular chain as a result of stretching treatment for imparting excellent transparency, heat resistance, and mechanical strength. An optical layered body that comprises a light-transmitting substrate having in-plane birefringence such as the above polyester film, when disposed on the surface of an image display device, may extremely decrease the anti-reflection properties on the surface of the optical layered body and may decrease the bright-field contrast.